The present invention relates to scientific instruments and particularly to a method and apparatus for measuring the lattice parameters of a single crystal material while that material is undergoing a transient shock wave.
The attainment of alternate materials phases at elevated static pressures has historically involved applied stresses in uniaxial, tetrahedral, or isotropic geometries. Transient high pressures have been applied to materials in the form of shocks. Shocks have resulted in changes in metallurgical properties, including phase changes, as determined by post-shock analysis.
A pressure field which is isotropic is called hydrostatic, since fluids (water) cannot support a shear stress and therefore support only isotropic pressure. However, this need not be so in crystalline solids, in which pressures can be directional. The details of nonhydrostatic pressures can be important to phase transitions, as in germanium, which transforms near 100 kbar with an applied uniaxial stress without a shear component, but at about 67 kbar in the presence of an additional shear component.
However, for solid-solid phase transitions, by whatever means of applying the elevated pressure, it has not been possible to directly determine the lattice parameters during the transition itself on any time scale below milliseconds. Also, the attainment of controlled strains in arbitrary directions has not heretofore been possible.
Lasers have been infrequently used in the past to thermally induce solid-solid phase changes in materials. In contrast, the body of work on laser-thermally-induced melting or annealing is very large. The effects of laser-induced shocks have been studied in certain aluminum alloys; metallurgical properties such as hardness and fatigue strength were found to be advantageously modified by laser-induced shocks. The mechanism for the production of shocks by lasers involves the ejection of mass from the surface of a material irradiated by the laser, which by conservation of momentum produces a pressure wave going into the volume of the material. Laser-produced shocks have been applied to solid materials in uniaxial geometries, although plasmas have also been shocked with cylindrical and spherical symmetry. Present laser systems built for a high degree of symmetry and uniformity of illumination are overpowered for the materials processing application, since they typically generate such high shock pressures that the temperature is raised sufficiently to induce melting and generate plasma.